combined or single indexes?

[Bryan Cantwell]

Is there a benefit to a combined index on a table? Or is multiple single
column indexes better?

If I have table 'foo' with columns a, b, and c. I will have a query
like:
select c from foo where a in (1,2,3) and b < 12345;

Is index on a,b better in any way than an a index and a b index?
An explain with one index sees it but doesn't use it (only the where) 
and having 2 indexes sees both and uses the one on b. 

Am I right to think that 2 indexes are better than one combined one?

thx,
Bryancan